The first book below is an overview (available online to holders of an Oxford SSO)
The second book is free to anyone as it has been published Open Access.
The Law Bod's print collection on Japanese law is predominantly in English (or other European languages).
Shelf marks begin Japan They are on Floor 1, one floor down from the level at which you enter the library.
The Law Bod has quite a strong collection of comparative law books, where Japan may be one of the jurisdictions examined. Books in thise collection have shelf marks beginning General. The collection is on Level 2, the level at which you enter the library.
SOLO searches will probably reveal that the Bodleian Japanese Library's collection also has material to support your studies for this jurisdiction.